JP H07-337007 and JP 3250217 (U.S. Pat. No. 6,301,131) disclose conventional switching power supply devices. Some switching power supply devices perform a current feedback control in a case that a high frequency switching operation is required for size reduction of an inductance. In this case, it is necessary in the current feedback control to mask a current signal for a predetermined period to separate a current signal from noise in a control circuit. The switching power supply device of this type is configured to be driven at a duty, which is higher than a predetermined duty irrespective of magnitudes of input and output voltages and a load current.
In an extreme situation, the load current flowing to an output side decreases to be zero and hence a discontinuous operation is necessitated correspondingly. However, in a case that the switching power supply device has a low-side circuit configuration by diode rectification, it cannot perform the discontinuous operation. It is therefore necessary to supply the current through a bleeder resistor in correspondence to a decrease of the load current thereby stabilizing load operation.
Since the bleeder resistor for supplying the load current is an addition of a load, the current flowing in the bleeder resistor causes loss of power under a heavy load condition and causes heat generation.